1. Field of the Invention
This invention relates to welding and more specifically to an apparatus for shielding a weld zone and suppressing plasma formation above a weld.
2. Description of the Prior Art
Welding by means of a beam of energy, such as a laser beam, is well known to the art. Many investigators have reported results using low or high power continuous or pulsed, laser beams to weld thin materials. A method of obtaining deep penetration welds in thick materials using carbon dioxide lasers is disclosed in U.S. Pat. No. 3,860,784 issued Jan. 14, 1975 to C. O. Brown et al. and held with the present application by a common assignee.
The generation of deep penetration welds is hindered by the problem of gas breakdown in the path of the beam. The resulting plasma defocuses and absorbs the beam which can seriously reduce the efficiency of producing welds. A mechanism of plasma generation by focusing a carbon dioxide laser beam on a solid surface is discussed in "Applied Physics Letters" Vol. 22, No. 10 at page 500 et seq. and the effects of the plasma on laser beam propagation is discussed in "Journal of Applied Physics" Vol. 46, No. 1 at page 138 et seq.
In dry air the breakdown threshold using a carbon dioxide laser beam is of the order of 10.sup.9 watts per square centimeter. The presence of water vapor or vapors ejected from the workpiece can reduce this threshold to the order of 10.sup.6 watts per square centimeter. Thus, two kilowatts of laser power can cause ionization if the beam is focused to a spot approximately 0.5 millimeter in diameter. The plasma produced by the ionization of the gas above the workpiece attenuates the beam and reduces the amount of power reaching the workpiece thereby seriously reducing the efficiency of producing a weld or even eliminating the generation of a weld. Efficient and practical deep penetration welding with laser beams requires the use of lasers having minimum power thresholds of 2 kilowatts.
Methods of solving the problem of beam attenuation by the plasma are known in the art. The introduction of a suppression gas having a high ionization potential such as nitrogen or helium across the laser beam path adjacent the workpiece can substantially raise the breakdown threshold. For a discussion of the technique of flowing a dry gas such as nitrogen across the beam path to prevent gas breakdown see a paper "Laser Welding Developments" published in "Welding Research Related to Power Plant" by Central Electricity Generating Board, London 1972. The concept is discussed also in U.S. Pat. No. 3,824,368. Flowing gas across a path of the welding beam has also been effective to minimize the contamination of beam focusing optics by the vapor and splatter ejected from the workpiece.
The problem of atmospheric contamination of the weld site associated with the more conventional welding processes such as oxi-actylene, MIG, TIG, submerged arc, electron beam, etc., is also present in the use of laser energy to weld metals. Atmospheric contamination of the weld site can be alleviated by welding in a nonreactive environment such as vacuum or inert gas. The disadvantages of welding in vacuum include limitations on the size and shape of the workpiece which can be accommodated as well as the time consumed in creating the vacuum conditions.
Welding in an inert gas environment utilizing either a total immersion technique or a trailer shield provides an alternate to welding in a vacuum. The trailer shield supplies a blanket of nonreactive gas such as argon, to the area of the workpiece over the fusion zone. The nonreactive gas covers the weld puddle and the weld zone preventing atmospheric gases from interacting with the material and contaminating the weld. The trailer shield is generally positioned such that nonreactive gas is not supplied much forward of the weld puddle.
Current laser welding practice is to flow the suppression gas in the direction of weld advance or at an angle thereto.
For a more detailed discussion of the use of suppression gas and cover gas to obtain improved welds see U.S. Pat. No. 4,000,392 entitled "Fusion Zone Purification by Controlled Laser Welding," filed on Dec. 9, 1975 and held with the present application by a common assignee.